1. Field of the Invention
The invention relates to electric power steering apparatuses.
2. Description of the Related Art
Conventionally, electric power steering apparatuses are known which perform steering wheel return control of applying torque in a returning direction of a steering wheel in addition to assist control of applying torque in a steering direction (see Japanese Patent Application Publication Nos. 2003-291842 (JP 2003-291842 A) and 2004-82856 (JP 2004-82856 A)). The steering wheel return control is performed in order to improve steering performance at the time of returning the steering wheel to a neutral position. In the electric power steering apparatus described in JP 2003-291842 A, a vehicle is estimated to be in a straight traveling state based on the vehicle speed, steering torque, steering angular velocity, and yaw rate, and a relative steering angle (relative rudder angle) at the time the vehicle is estimated to be in the straight traveling state is estimated as a neutral angle. An absolute steering angle (absolute rudder angle) is computed based on the neutral angle and the relative steering angle, and steering wheel return torque is computed based on the absolute steering angle by a steering wheel return control unit. Command torque is computed by adding the steering wheel return torque, inertia torque, and damper torque to assist torque computed by an assist control unit, and a motor is driven and controlled based on a command current value according to the command torque.
In electric power steering apparatuses in which steering wheel return torque or its corresponding compensating current value is computed based on an absolute steering angle as in the electric power steering apparatus described in JP 2003-291842 A, the steering wheel return torque and its corresponding compensating current value are not computed until it is allowed to compute the absolute steering angle, namely until a vehicle is estimated to be in a straight traveling state. Accordingly, the steering wheel return torque and its corresponding compensating current value are not reflected in command torque and a command current value before it is allowed to compute the absolute steering angle. After it is allowed to compute the absolute steering angle, the steering wheel return torque and its corresponding compensating current value are reflected in the command torque and the command current value.
Therefore, there is a difference in steering feel between before and after it is allowed to compute the absolute steering angle. Specifically, returning movement to a neutral position of the steering is worse before it is allowed to compute the absolute steering angle than after it is allowed to compute the absolute steering angle. The driver therefore may have an uncomfortable steering feel before and after it is allowed to compute the absolute steering angle.